EXCHANGE 


15he 
Influence  of  Certain  Electrolytes 

ON  THE 

Composition  of  Saturated 
Bredig  Gold  Sols 


DISSERTATION 

SUBMITTED    IN    PARTIAL    FULFILMENT    OF    THE    REQUIRE- 
MENTS FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 
IN    THE    FACULTY    OF    PURE    SCIENCE    IN 
COLUMBIA    UNIVERSITY 


BY 

LEWIS  BENJAMIN   MILLER,  B.A.,  M.S. 


NEW  YORK  CITY 
1922 


Dedicated    to 
PROFESSOR  ELBERT  WILLIAM  ROCKWOOD 


ACKNOWLEDGMENT 

The  author  wishes  to  express  to  Professor  H.  T.  Beans  his 
sincere  thanks  for  the  suggestion  of  the  problem  and  for  generous 
assistance  and  advice  throughout  the  course  of  the  investigation. 

He  also  wishes  to  thank  the  other  members  of  the  Chemistry 
Department  for  their  co-operation  and  assistance. 


The  Influence  of  Certain  Electrolytes 

ON  THE 

Composition  of   Saturated 
Bredig  Gold  Sols. 


INTRODUCTION 

The  fact  that  certain  colloidal  dispersions  were  stable  only 
in  the  presence  of  small  amounts  of  electrolytes  was  first  ob- 
served by  Graham1,  who  found  that  ferric  hydroxide  and  alum- 
inium hydroxide  sols  could  not  be  entirely  freed  from  chloride, 
no  matter  how  long  the  solutions  were  dialyzed.  A  great  deal 
of  work  has  since  been  done  on  these  and  other  hydrosols  show- 
ing that  the  presence  of  electrolyte  is  necessary  to  stabilize  the 
colloid2. 

The  work  of  Picton  and  Linder18,  Jordes  and  Kauter19,  Lot- 
termoser20,  Svedberg21,  and  von  Weimarn22  has  led  to  the  de- 
velopment of  the  hypothesis  that  the  particles  of  a  hydrosol  con- 
sists of  a  complex  between  a  substance  of  very  low  solubility 
and  an  ion  of  an  electrolyte,  the  presence  of  which  is  necessary 
to  the  stability  of  the  sol  and  to  which  the  sol  particle  owes  its 
electrical  charge. 

While  the  application  of  this  chemical  or  complex  theory 
to  inorganic  hydrophilic  dispersions  has  been  general,  hydro- 
phobic  dispersions,  particularly  those  of  the  noble  metals  pre- 
pared by  electrical  dispersion,  have  been  considered  to  be  es- 
sentially different  and  to  require  no  stabilizing  electrolyte3. 
Bredig4,  however,  recommended  the  use  of  .001  N  sodium 
hydroxide,  while  Whitney  and  Blake5  state  that  "a  more  stable 
and  more  concentrated  sol  could  be  obtained  by  sparking  gold 
electrodes  in  the  presence  of  a  solution  of  hydrochloric  acid  hav- 
ing a  specific  conductivity  of  200xlO"6  Mhos.  That  hydrophobic 
dispersions  probably  also  require  an  electrolyte  for  their 
stabilization  was  indicated  by  the  work  of  Beans  and  Eastlack5, 
who  found  that  when  carefully  distilled  water  ("conductivity 
water")  was  used  as  the  dispersion  medium,  stable  gold  sols 
could  not  be  obtained  by  the  Bredig  method.  The  addition  to  the 
water  of  certain  electrolytes  in  concentrations  varying  from 

502085 


0.00005  to  0.005  normal  resulted,  however,  in  the  formation  of 
stable  gold  sols.  Furthermore,  that  these  sols  were  stabilized  by 
the  formation  of  some  such  complex  as  is  assumed  to  be  neces- 
sary in  the  case  of  hydrophilic  dispersions  was  strongly  indicated 
by  the  fact  that  only  those  electrolytes  furnishing  a  negative  ion 
capable  of  combining  with  gold  to  form  a  compound  stable  in 
aqueous  solution  are  effective.  And,  finally,  this  work  demon- 
strated that  the  complex  is  formed  between  the  stabilizing  ion 
and  metallic  gold  since  in  all  cases  the  solutions  were  found 
to  be  free  from  gold  ion. 

Beans  and  Beaver6  found,  in  arcing  gold  electrodes  in  solu- 
tions of  the  nature  and  concentration  described  above,  that  the 
concentration  of  the  dispersed  gold  increased  with  time  of  arcing 
tip  to  a  certain  limit  depending  on  the  concentration  of  the 
electrolyte.  If  the  process  was  continued  beyond  this  limit  the 
concentration  decreased  and  the  sol  was  entirely  coagulated  by 
a  comparatively  short  period  of  arcing.  They  demonstrated  also 
that  centrifuging  for  one  hour  in  a  centrifuge  capable  of  de- 
veloping a  force  1200  times  gravity  gave  a  sol  of  constant  gold 
content.  This  shows  that  this  force  is  inadequate  to  precipitate 
the  colloidal  particles,  while  it  does  remove  the  coarse  suspended 
particles. 

The  important  discovery  of  Beans  and  Beaver  that  the 
limiting  concentration  of  gold  in  a  Bredig  sol  depends  upon  the 
concentration  of  the  electrolyte  demanded  quantitative  study  and 
it  is  the  result  of  this  investigation  that  forms  the  subject  of  this 
thesis. 


APPARATUS  AND  MATERIALS  USED 

All  solutions  were  made  up  in  water  having  a  specific  con- 
ductivity of  1.46  to  l.OOxlO"6  imhos.  The  hydrochloric  and 
hydrobromic  acids  were  the  constant  boiling  point  acids  pre- 
pared in  quartz  apparatus  from  Baker  and  Adamson's  c.  p. 
products.  Baker's  c.  p  potassium  chloride  was  twice  re-crystal- 
lized from  conductivity  water  and  fused  in  a  platinum  dish. 
Baker's  c.  p.  sodium  chloride,  potassium  bromide,  and  potassium 
iodide  were  each  re-crystallized  three  times  from  conductivity 
water  and  dried  at  110°  in  a  Freas  electric  oven.  The  cesium 
chloride  was  prepared  from  five'  times  re-crystallized  cesium 
nitrate,  which  gave  no  test  for  rubidium  upon  spectroscopic  ex- 


animation,  by  the  method  of  H.  L.  Wells7.  After  purification  the 
salt  gave  a  negative  test  for  iodine  when  a  portion  of  it  was 
shaken  with  chloroform,  after  acidifying  with  hydrochloric  acid 
and  adding  a  few  drops  of  sodium  nitrite.  Kahlbaum's  c.  p. 
rubidium  chloride,  found  to  be  spectrpscopically  free  from  cesium 
salts,  was  not  further  purified.  Baker's  c.  p.  lithium  chloride  was 
gently  ignited  in  a  platinum  dish  to  remove  water,  dissolved  in 
absolute  alcohol  and  filtered  to  remove  sodium  and  potassium 
chlorides.  The  alcoholic  solution  was  then  evaporated  to  dryness 
and  gently  ignited.  The  solution  obtained  by  dissolving  this 
product  in  water  was  acid  to  phenolphthalern.  Sodium  hydrox- 
ide solution,  prepared  from  Eimer  and  Amend's  c.  p.  product 
which  was  prepared  from  metallic  sodium,  was  made  up  by 
filtering  a  saturated  aqueous  solution  of  the  base  through  a  dry 
filter  paper,  which  separated  all  carbonate  and  chloride,  and 
diluting  with  water. 

All  solutions  were  made  up  to  approximately  0.02  molar  con- 
centration and  standardized.  The  sodium  hydroxide  was  stand- 
ardized against  Bureau  of  Standards  pure  benzoic  acid.  The 
hydrochloric  and  hydrobromic  acids  were  standardized  against 
the  sodium  hydroxide.  Potassium  chloride,  bromide,  and  iodide, 
and  the  chlorides  of  sodium,  rubidium  and  cesium  were  standard- 
ized by  evaporating  a  definite  volume  to  dryness  in  a  weighed 
platinum  dish,  drying  for  several  hours  in  an  electric  oven  at 
110°C,  and  finally  igniting  gently  on  an  air  bath,  cooling  and 
weighing.  The  lithium  chloride  was  standardized  by  evaporat- 
ing a  definite  volume  of  the  solution  in  a  platinum  dish  in  the 
presence  of  an  excess  of  sulphuric  acid  and  finally  igniting  on  an 
air  bath,  cooling  and  weighing  as  lithium  sulphate. 

The  gold  wire  and  arcing  apparatus  were  the  same  as  that 
used  by  Beans  and  Eastlack8,  with  the  addition  of  a  stifring 
device. 

The  centrifuge  used  for  freeing  the  sols  of  coarse  particles 
was  a  "type  three  centrifuge"  of  the  International  Equipment 
Co.,  with  rotating  arms  two  feet  in  diameter  and  with  a  speed 
2000  r.p.m. 

The  burette,  pipettes,  volumetric  flasks,  and  set  of  weights 
used  were  all  standardized  by  the  usual  methods. 


METHOD  OF  PROCEDURE 

All  solutions  for  arcing    were    made    up    by    diluting    the 

3 


standard.  The  solution  was  then  placed  in  a  pyrex  beaker  and 
surrounded  by  an  ice  bath  which  kept  the  temperature  in  the 
neighborhood  of  25°  to  30°  C.  during  the  arcing,  the  solution  be- 
ing stirred  vigorously  by  a  glass  stirrer  connected  to  a  motor. 
When  the  condition  of  saturation  of  the  sol  with  respect  to 
colloidal  gold  was  approached,  portions  of  the  solution  were  re- 
moved at  short  intervals  of  time  and  placed  in  well  steamed  non- 
sol  bottles.  This  was  continued  until  the  sol  precipitated.  The 
samples  were  then  placed  in  glass  centrifuge  tubes  and  centri- 
fuged  for  one  hour  at  1200  times  gravity.  A  definite  volume  of 
the  upper  layer,  free  from  coarse  suspension  particles,  was  then 
pipetted  off  carefully,  precipitated  with  a  few  drops  of  hydro- 
chloric acid,  filtered  through  an  ashless  filter  paper,  burned  and 
weighed  in  a  porcelain  crucible.  A  volume  of  sol  was  analyzed 
which  yielded  at  least  50  milligrams  of  gold.  From  the  highest 
value  given  by  a  set  of  samples  the  amount  of  gold  in  moles  per 
liter  was  calculated. 

Numerous  points  in  each  electrolyte  were  duplicated  by  mak- 
ing a  fresh  dilution  from  the  standard  and  running  the  entire 
procedure  again.  In  no  case  (except  in  sols  prepared  in  cesium 
chloride  solution)  was  the  variation  in  the  maximum  quantity  of 
gold  determined  by  the  two  duplicates  greater  than  two  percent, 
of  the  total  gold.  In  cesium  chloride  solutions,  however,  it  was 
much  more  difficult  to  determine  when  the  condition  of  saturation 
was  being  reached  and  some  of  the  duplicates  varied  as  much  as 
five  percent,  from  each  other. 

Potassium  chloride  was  the  first  electrolyte  to  be  tried  and 
it  was  fairly  completely  investigated  throughout  the  range  for 
colloid  stability.  The  /one  of  high  concentrations  for  gold  col- 
loid seemed  the  more  important  and  interesting,  however,  So 
in  the  remaining  electrolytes  the  region  of  high  concentration 
for  gold  was  carefully  investigated;  the  regions  of  low  concen- 
tration, only  in  a  general  way. 

In  the  following  tables,  obtained  as  outlined  above,  the  con- 
centration of  electrolyte  is  expressed  in  mols  per  liter,  the  con- 
centration of  gold  in  atomic  weights  per  liter. 


TABLE 

I  H  Br 

Cone,  electro- 

Cone. Gold 

lyte  mols  per 

at.  wt. 

litre. 

per  1. 

0.00020 

0.0018 

0.00050 

0.0022 

0.00080 

0.0048 

0.00100 

0.0020 

0.00110 

0.0036 

0.00120 

0.0045 

0.00130 

0.0041 

0.00150 

0.0018 

0.00180 

0.0036 

0.00200 

0.0031 

0.00220 

0.0029 

0.00250 

0.0018 

TABLE 

III  K  Cl 

Cone,  electro- 

Cone. Gold 

lyte  mols  per 

at.  wt. 

litre. 

per  1. 

0.0000125 

0.0012 

0.000025 

0.0014 

0.00005 

0.0031 

0.00010 

0.0047 

0.00060 

0.0105 

0.00100 

0.0125 

0.00120 

0.0178 

0.00130 

0.0218 

0.00140 

0.0211 

0.00160 

0.0161 

0.00200 

0.0181 

0.00230 

0.0179 

0.00250 

0.0173 

0.00300 

0.0122 

0.00500 

0.0107 

0.00650 

0.0061 

0.00800 

0.0062 

0.01000 

0.0048 

0.01200 

0.0044 

0.01500 

slight 

0.02000 

none 

TABLE  II  K  Br 


Cone,  electro- 

Cone.  Gold 

•lyte  mols  per 

at.wt. 

litre. 

per  1. 

0.00010 

0.0046 

0.00080 

0.0086 

0.00100 

0.0103 

0.00120 

0.0162 

0.00130 

0.0146 

0.00160 

0.0102 

0.00200 

0.0053 

0.00300 

0.0028 

0.00500 

0.0006 

TABLE  IV 

H   Cl 

Cone,  electro- 

Cone. Gold 

lyte  mols  per 

at.wt. 

litre. 

per  1. 

0.00010 

0.0043 

0.00060 

0.0053 

0.00100 

0.0082 

0.00110 

0.0095 

0.00120 

0.0108 

0.00130 

0.0093 

0.00140 

0.0076 

0.00180 

0.0066 

0.00200 

0.0073 

0.00220 

0.0061 

0.00300 

0.0046 

0.00500 

0.0016 

TABLE  V  Li  Cl 


Cone,  electro- 

h  te  mols  per 

litre. 

0.00050 

0.00100 

0.00120 

0.00130 

0.00140 

0.00160 

0.00180 

0.00210 

0.00250 

0.00290 

0.00305 

0.00320 

0.00360 

0.00700 

TABLE 
Cone,  electro- 
lyte mols  per 
litre. 

0.00050 

0.00100 

0.00110 

0.00120 

0.00130 

0.00150 

0.00170 

0.00180 

0.00220 

0.00260 

0.00700 


Cone.  Gold 
at.  wt. 
per  1. 
0.0125 
0.0178 
0.0213 
0.0225 
0.0210 
0.0205 
0.0203 
0.0202 
0.0206 
0.0210 
0.0207 
0.0209 
0.0201 
0.0172 

VII  Rb  Cl 
Cone.  Gold 
at.  wt. 
per  1. 
0.0112 
0.0150 
0.0151 
0.0147 
0.0190 
0.0111 
0.0128 
0.0131 
0.0100 
0.0088 
0.0047 


TABLE  VI 

Na   Cl 

Cone,  electro- 

Cone. Gold 

lyte  mols  per 

at.wt. 

litre. 

per  1. 

0.00050 

0.0140 

0.00080 

0.0165 

0.00100 

0.0175 

0.00120 

0.0182 

0.00130 

0.0258 

0.00140 

0.0206 

0.00160 

0.0176 

0.00180 

0.0216 

0.00190 

0.0203 

0.00200 

0.0201 

0.00210 

0.0222 

0.00240 

0.0204 

0.00280 

0.0191 

0.00500 

0.0107 

TABLE  VIII 

Cs   Cl 

Cone,  electro- 

Cone, (iold 

lyte  mols  per 

at.wt. 

litre. 

per  1. 

0.00060 

0.0132 

0.00080 

0.0156 

0.00090 

0.0179 

0.00095 

0.0167 

0.00100 

0.0142 

0.00105 

0.0172 

0.00110 

0.0184 

0.00115 

0.0140 

0.00120 

0.0131 

0.00130 

0.0150 

0.00135 

0.0161 

0.00140 

0.0148 

0.00150 

0.0133 

0.00170 

0.0138 

0.00200 

0.0121 

0.00400 

0.0077 

TABLE  IX  K  I 
Cone,  electro-  Cone.  Gold 

lyte  mols  per  at.  wt. 

litre.  per  1. 

0.00100  0.00013  very  dilute  purple  sol. 

0.00130  Dilute  reddish,  purple  sol.     Slightly  more  concen- 

trated than   .001. 

0.00150  Dilute   reddish,   purple   sol. 

0.00200  Very   dilute   purple    sol. 

Above  and  below  the  above   concentrations  no 
sols,  stable  at  1200  times  gravity,  were  formed. 

The  curves  in  figures  I,  II,  and  III  were  obtained  by  plotting 
the  concentration  of  electrolyte  expressed  in  mols  per  liter  as 
ordinates  against  concentration  of  colloidal  gold  expressed  in 
atomic  weights  per  liter  as  abscissa. 

These  curves  perhaps  represent  the  resultant  of  two  forces 
acting  upon  the  gold  particle,  namely,  the  stabilizing  effect  of  the 
negative  ion  and  the  precipitating  effect  of  the  positive  ion.  If 
we  consider  the  stabilizing  effect  of  the  chloride  ion  to  be  a  con- 
stant for  the  various  alkali  chlorides,  then  it  is  evident  that  the 
precipitating  power  of  the  cations  increase  in  the  following  order, 
Li,  Na,  K,  Rb,  Cs,  H.  It  is  to  be  observed  that  this  is  the  same 
order  as  the  precipitating  value10  found  for  these  ions  when 


electrolytes  containing  them  are  added  to  negative  colloids  to 
produce  precipitation.  It  is  also  in  the  same  order  as  the  mo- 
bility9 of  the  ions,  which  increase  from  lithium  ion  to  hydrogen 
ion.  It  is  the  inverse  order  of  the  degree  of  hydration11  of  the 
ions  and  the  ionic  radii1-.  Lor  the  alkali  metals  this  is  also  the 
same  order  as  increase  of  atomic  weights  and  metallic  properties. 

The  stabilizing  action  of  the  negative  ions  decrease  in  the 
following  order :  chloride,  bromide,  and  iodidej^~This  is  the  same 
order  as  the  decrease  of  precipitating  power  upon  positively 
charged  colloids  when  electrolytes  containing  these  ions  are  added 
to  the  colloids13.  It  is  also  the  same  order  as  the  decrease  in 
ionic  hydration11,  non-metallic  properties  of  the  elements,  and 
the  stability  of  the  corresponding  gold  halides.  It  is  the  inverse 
order  for  these  ions  of  the  stabilizing  effect  upon  negative  col- 
loids 14  when  electrolytes  containing  these  ions  are  added  to 
negatively  charged  colloids  for  the  purpose  of  producing  pre- 
cipitation, and  of  the  decrease  of  atomic  weights.  The  ionic 
radii15  and  the  mobilities18  of  the  chloride,  bromide,  and  iodide 
ions  are  nearly  identical. 

With  the  exceptions  of  the  curves  for  hydrobromic  acid  and 
cesium  chloride,  the  first  and  highest  maximum  point  for  gold 
concentration  of  each  curve  occurs  at  a  concentration  of  electro- 
lyte varying  between  0.0012  and  0.0013  molar.  For  the  two  ex- 
ceptions the  second  maximum  occurs  at  these  concentrations ; 
the  first  maximum  occurs  between  0.0008  and  0.0009  molar  con- 
centration of  electrolyte. 

If  we  assume  that  there  are  two  forces  acting  upon  the  col- 
loid— the  protective  action  of  the  ion  of  like  charge  to  the  col- 
loid and  the  precipitating  action  of  the  oppositely  charged  ion, 
(Bredig  gold  sols  are  negatively  charged) — then  the  peculiar 
character  of  the  curves  can  be  explained  in  one  of  the  following 
ways: 

(1)  The   negative   ions   possess   a   high    stabilizing   power 
through  certain  narrow  ranges  of  concentration  lying  within  the 
limits  of   concentration   through   which   the   colloids   are   stable. 
This  results  in  the  formation  of  one  or  more  maxima  when  gold 
concentration  is  plotted  against  electrolyte  concentration. 

(2)  The  positive  ions  show  a  marked  loss  of  precipitating 
power  through  certain  narrow  ranges. 

(3)  Or  a  combination  of  these  two  opposing  actions  occurs. 
The  first  hypothesis  is  supported  by  the  fact  that  the  maxima 

8 


COMC    GOLD  IN  AT  WT 

Ci  ft 


CONC  GOLD  IN  AT  WT 


^-. 


*£. 


CONC.  OF  GOLD  IN  AT.WT. 


ATOMIC  WEIGHTS  GOLD  PER  MO LS  KCI 


CONC  OF  GOLD  IN  AT.WT.  PER  LITER 


10 


for  the  chlorides  of  hydrogen  and  of  the  alkali  metals  lie  in  the 
same  general  range  of  electrolyte  concentration  (.0012  —  .0013 
molar)  and  by  the  general  similarity  of  the  curves.  It  is  further 
supported  by  the  fact  that  the  order  in  which  the  curves  lie  for 
electrolytes  containing  the  same  positive  ion  varies  in  the  same 
or  inverse  order  as  the  variation  of  certain  properties  of  the 
negative  ions.  It  is  contradicted  by  the  fact  that,  though  the 
general  similarity  of  the  curves  is  evident,  the  details  of  the 
curves,  number  and  location  of  the  maxima  vary  with  different 
compounds  yielding  the  same  negative  ion,  as  well  as  by  the  fact 
that  the  order  in  which  the  curves  lie  for  the  same  negative  ion 
varies  in  the  same  or  inverse  order  as  certain  properties  of  the 
positive,  ions. 

The  reverse  of  the  above  arguments  applies  to  the  second 
hypothesis.  And  in  this  connection  it  is  interesting  to  note  that 
Fales  17has  found  that  the  apparent  degree  of  dissociation  of  sev- 
eral acids,  including  hydrochloric  acid,  as  calculated  from  elec- 
tromotive force  measurements  by  the  hydrogen  electrode  reaches 
a  maximum  at  about  0.001  normal  concentration  of  the  acids. 
This  peculiarity,  he  thinks,  may  possibly  be  ascribed  to  hydration 
of  the  hydrogen  ion.  The  concentration  at  which  this  maximum 
occurs  is  almost  exactly  the  same  as  that  at  which  the  gold  sols 
were  in  general  found  to  be  most  stable. 

In  the  face  of  the  conflicting  evidence  just  cited,  the  last  of 
the  three  hypotheses  seems  the  most  reasonable.  In  brief,  there- 
fore, the  data  gathered  in  this  investigation  seems  to  indicate  that 
the  effects  are  the  resultant  of  the  precipitating  action  of  the  posi- 
tive ion  and  the  stabilizing  action  of  the  negative  ion. 

If,  from  the  values  for  the  numbers  of  atomic  weights  of 
gold  present  in  the  saturated  sol  and  the  molar  concentration  of 
electrolyte,  the  ratio  of  the  numbers  of  atomic  weights  of  gold  to 
concentration  of  electrolyte  be  plotted  as  ordinates  against  con- 
centration of  electroylte  as  abscissa,  a  curve  which  is  very  similar 
to  an  hyperbola  is  produced.  This  ratio  increases  rapidly  as 
the  concentration  of  electrolyte  diminishes,  varying  for  potassium 
chloride  between  96.8  atoms  of  gold  per  molecule  of  electrolyte 
for  0.0000125  molar  K  Cl  and  0.48  atoms  of  gold  per  molecule 
of  electrolyte  for  0.01  molar  K  Cl.  In  the  absence  of  information 
as  to  whether  all  the  chloride  ions  act  as  stabilizing  units  it  is 
uncertain  that  this  method  of  calculation  is  significant.  It  may, 
therefore,  be  taken  as  giving  the  minimum  value  for  the  average 

11 


number  of  atoms  of  gold  which  may  be  attached  to  a  chloride 
ion  in  a  saturated  sol  for  each  concentration  of  electrolyte. 

The  values  obtained  as  described  above  have  been  plotted  in 
Figure  I,  together  with  the  complete  saturation  curve  with  re- 
spect to  gold  for  that  electrolyte.  The  data  is  given  in  Table  X. 

TABLE   X 

Mols  of  K  Cl  per  liter          Atoms  of  gold  per  mol  of  K  Cl 

0.0000125  96.8 

0.000025  54.4 

0.00005  61.0 

0.00010  47.4 

0.00060  17.6 

0.00100  12.5 

0.00120  14.8 

0.00130  16.8 

0.00140  15.1 

0.00160  10.0 
0.00200  9.0 

0.00230  7.8 

0.00250  7.0 

0.00500  2.1 

0.00650  .9 

0.00800  .8 

0.01000  .5 

0.01200  .36 

0.01500  slight 

0.02000  none 

The  efficiency  of  the  arc  in  producing  colloidal  gold  varied 
for  different  electrolytes;  that  is,  the  time  of  arcing  in  different 
electrolytes  necessary  to  produce  a  sol  containing  a  definite 
amount  of  colloidal  gold  varied  considerably.  For  the  chlorides 
the  efficiency  varied  as  follows :  H  or  Li  greater  than  Na  greater 
than  Rb  greater  than  K  or  Cs.  The  efficiency  in  hydrogen  or 
lithium  chlorides  was  relatively  twice  as  great  as  in  potassium  or 
cesium  chlorides.  The  rate  at  which  the  gold  electrodes  were 
disintegrated  was  roughly  of  the  same  order.  The  efficiency  also 
varied  for  different  concentrations  of  the  same  electrolyte,  being 
greatest  in  the  more  dilute  solutions,  and  decreasing  as  the  con- 
centration of  electrolyte  increased. 

Though  nothing  conclusive  has  been  established  as  to  the 
process  by  which  colloidal  dispersions  are  formed  by  the  Bredig 
method,  a  study  of  the  experimental  results  and  conclusions  of 
most  of  the  previous  investigators  in  this  field  points  to  the 

12 


process  as  being  a  purely  thermo-mechanical  one  in  which  the 
metal  is  volatilized  by  the  hot  arc  and  then  condensed  by  the  cold 
dispersion  medium  to  fine  solid  particles  which  remain  suspended 
in  the  liquid.  That  the  process  was  not  as  simple  as  this  was 
indicated  by  the  work  of  Beans  and  Eastlack5  who  showed  that  a 
definite  kind  and  concentration  of  electrolyte  was  necessary  to 
produce  a  stable  colloidal  suspension.  The  fact  that  the  efficiency 
of  the  arc,  kept  under  as  constant  conditions  as  possible,  varies 
so  greatly  in  different  electrolytes  points  to  the. same  conclusion. 
The  work  of  Beans  and  Beaver6  has  shown  that  the  stabilizing 
electrolyte  is  very  definitely  and  strongly  attached  to  the  gold 
particle  forming  a  complex  which  is  not  easily  broken  down  by 
mechanical  methods.  The  present  work  has  shown  that  each  con- 
centration of  electrolyte  possesses  a  definite  saturation  value  for 
gold  under  the  conditions  of  formation,  and  that  when  this  satura- 
tion value  is  exceeded  the  whole  system  becomes  unstable.  Each 
electrolyte  which  will  stabilize  gold  has  certain  peculiar  properties 
towards  stabilization.  All  of  these  facts  point  to  the  conclusion 
that  the  process  of  formation  is  something  more  than  a  merely 
thermo-mechanical  process. 

To  determine  whether  the  curves  represent  the  stability  range 
of  the  colloids  under  all  conditions  or  only  for  the  conditions  of 
preparation,  the  following  experiments  were  performed :  A  0.0013 
molar  solution  of  sodium  chloride  was  prepared  and  arced  until 
about  four-fifths  saturated  with  gold.  It  was  then  centrifuged 
and  upon  analaysis  was  found  to  contain  4.070  grams  of  gold  per 
liter  (maximum  value  of  gold  found  for  this  concentration  of 
electrolyte  is  5.088  grams  per  liter).  A  known  volume  of  the 
centrifuged  sol  was  placed  in  a  steamed  pyrex  beaker  which  was 
then  placed  in  a  vacuum  dessicator  over  sulfuric  acid  and  allowed 
to  evaporate  slowly  until  the  volume  was  reduced  about  half. 
By  this  process  the  concentrations  of  gold  and  electrolyte  were 
so  changed  as  to  bring  them  above  the  region  of  stability  as 
found.  During  the  process  long,  very  thin,  yellow  ribbons  of 
gold  formed  on  the  surface  of  the  sol  and  assumed  a  more  or  less 
feathery  structure,  appearing  very  much  like  crystals.  Under 
the  microscope  they  had  the  appearance  of  gold  leaf  and  were 
so  thin  that  light  passed  through  them.  A  portion  of  the  re- 
maining solution  was  removed,  centrifuged  in  the  usual  manner, 
the  upper  portion  pipetted  off  and  analyzed.  It  was  found  to 
contain  6.620  grams  of  gold  per  liter.  This  amount  is  greater 

13 


than  the  highest  point  on  the  sodium  chloride  curve  by  about 
thirty  percent. 

Secondly,  a  0.0012  molar  potassium  bromide  solution  was 
prepared  and  arced  until  nearly  saturated  with  gold.  After 
centrifuging  it  was  analyzed  and  found  to  contain  3.030  grams 
of  gold  per  liter  (maximum  value  of  gold  for  this  concentration 
of  electrolyte  is  3.204  grams  per  liter).  Three  parts  of  the  centri- 
fuged  sol  were  then  diluted  with  one  part  of  water,  placed  in  a 
well  steamed  non:sol  bottle  and  allowed  to  stand  for  twenty-four 
hours.  By  diluting  in  this  ratio  the  concentration  of  both  gold 
and  electrolyte  were  so  changed  as  to  bring  them  below  the  region 
of  stability  as  found.  After  standing,  the  sol  was  again  centri- 
fuged  and  analyzed.  It  was  found  to  contain  2.270  grams  of 
gold  per  liter. 

.  4  i 

x  2.270  =  3.027 


The  sol  is  thus  seen  to  contain,  within  experimental  error  of 
analysis,  the  same  amount  of  gold  per  liter  as  before 

From  these  experiments  it  is  evident  that  the  curves  shown 
here  represent  the  regions  of  stability  for  the  formation  of  these 
sols  under  the  experimental  conditions  described,  but  not  for 
changes  of  concentration  of  either  gold  or  electrolyte  which  may 
occur  after  the  formation  has  taken  place. 

SUMMARY 

It  has  been  shown  that : 

(1)  In  the  preparation  of  gold  sols  by  the  Bredig  method,, 
there  is  a  maximum  amount  of  gold  which  can  be  stabilized  by 
each  concentration  of  electrolyte. 

(2)  One  or  more  maxima  occur  in  each  curve  produced  by 
plotting  concentration  of  gold  against  concentration  of  electrolyte. 

(3)  The  first  and  highest  maximum  point  occurs  at  a  con- 
centration of  electrolyte  of  0.0012  to  0.0013  molar  with  two  ex- 
ceptions. 

(4)  The  curves  represent  the  regions  of  stability  for  the 
formation  of  these  sols  but  not  for  subsequent  changes  of  con- 
centration which  these  sols  may  undergo. 

(5)  The  process  of  formation  of  Bredig  gold  sols  cannot 
be  purely  thermo-mechanical. 

14 


BIBLIOGRAPHY 

.  and  Physical  Researches,  pp  582,  580  (1976). 

2See  Cassuto,  Der  Kolloide  Zustand  der  Moterie  (1913)  p 
190. 

3Freundlich,  Kapillarchemie  (1909)  p  323;  Cassuto,  p  213; 
and  Taylor,  Chemistry  of  Colloids  (1915)  p  110. 

4Anorganische  Fermente,  Dissertation,  Leipsig  (1901)  p  26. 

r'J.  A.  C.  S.  26,  1376  (1904). 

6D.  J.  Beaver,  Dissertation  for  Ph.D.,  Columbia  University. 

7Am.  Chem.  J.  26,  (1901)  265. 

8Loc.  cit.  p  2673. 

9Washburn,  J.  A.  C.  S.  37,  (1915)   694. 

10Galecki,  Zeitsch.  Anorg.  Chem.  74,   (1912)   174. 

"Smith,  J.  A.  C.  S.  37,  (1915)  722, 

12Mellor,  Comprehensive  Treatise  on  Inorg.  and  Theoretical 


Chem.  vol.  ii,  p  461. 

13Freundlich,  Kapiliarchemie. 

"Linder  and  Picton,  J.  Chem.  Soc.,  67,  63. 

15Mellor,  ibid.,  p  65. 

1BMellor,  ibid.,  p  194. 

1TRobertson,  Dissertation  for  Ph.D..  Columbia  University. 

18J.  Chem.  Soc.  61.  (1892)  114;  87,  (1905)  1906. 

19Z.  Anorg.  Chem.,  35,  (1903)  16. 

20Ibid.,  44,   (1905)   200. 

21Summary    and    earlier    references,    Z.    phys.    Chem.,    70, 

(1910)   239. 

22Kolloid  Z.,  2,   (1907)   142. 

23For  a  summary    of    this    work    see    Beans    and    Eastlack, 
J.A.C.S.  37,  2667;  and  Makhopadhyaya,  J.A.C.C.,  37,  292. 


15 


VITA 

Lewis  Bejamin  Miller  was  born  near  Parkersburg,  Iowa,  on 
June  24,  1896,  and  attended  the  grade  and  high  schools  of  that 
city.  He  received  the  degree  of  B.A.  from  the  State  University 
of  Iowa,  Iowa  City,  Iowa,  in  June,  1919.  He  attended  the  gradu- 
ate school  at  that  place  from  June,  1919,  until  August,  1920,  at 
which  time  he  received  the  degree  of  M.S.  During  the  three 
years  1918  to  1920  he  was  Assistant  in  Chemistry  at  the  State 
University  of  Iowa.  From  September,  1920,  until  the  present 
he  has  attended  the  graduate  school  of  Columbia  University. 
He  was  laboratory  assistant  in  Inorganic  Chemistry  from  Sep- 
tember, 1920,  till  June,  1921.  and  Assistant  in  Chemistry  from 
July,  1921,  until  the  present. 


16 


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